A system of this generic type is used in of series 6030 and 7030 John Deere tractors, in which a charging oil pump continuously conveys the hydraulic fluid against the force of gravity from a working reservoir formed by a differential casing into a higher auxiliary reservoir for interim storage, wherein the hydraulic fluid is fed from the auxiliary reservoir via a controllable high-pressure pump into a hydraulic circuit for operating hydraulic vehicle units. Hydraulic fluid no longer needed by the vehicle units is conducted back into the working reservoir. The hydraulically operated vehicle units are, in particular, a steering and braking system, as well as agricultural attachments that can be mounted on the tractor and have hydraulic actuating cylinders or the like. Depending on the hydraulic fluid consumption of the vehicle units, there can be more or less pronounced fluctuations of the fluid level in the working reservoir.
Since a pressure on the order of several bar is present at the output side of the charging oil pump while it is conveying hydraulic fluid, there are also increased requirements for pressure load capabilities of the auxiliary reservoir. Due to the associated extra construction expense, these requirements lead to corresponding additional costs. If an increased withdrawal of hydraulic fluid occurs during operation of the vehicle units, there is also the possibility that the working reservoir may be completely emptied in the direction of the auxiliary reservoir due to the continuous operation of the charging oil pump, and therefore the charging oil pump may continue to run “dry” or un-lubricated. The latter may not only have a negative effect on the service life of the charging oil pump, but can also lead to an interruption of the lubrication for the differential gear unit present in the differential casing.